Myopia is defined as the state wherein parallel rays of light entering the eye are brought to focus before the retina.
Myopia generally includes the following two kinds. One is axial myopia. This is caused by abnormally long axis of the eye when lens power is normal, and severe myopia is often found in this kind. The axial myopia generally starts from infancy and the axis of the eye gradually grows and abnormalities are developed in fundus oculi. Specifically, atrophy of retina and chorioidea, abnormal increase and decrease of pigments, clouding of vitreous body, retinal hemorrhage to result in detachment of the retina and even losing eyesight can be observed.
The other is refractive myopia. This is caused by abnormal curvature of cornea or acquired increase in the thickness of the lens due to reading, close work for the eye such as VDT (visual display terminal) work, etc. to result in too strong a power of cornea and lens. Included therein is pseudomyopia which is caused by unfixed state of continuing and growing contraction of the ciliary smooth muscle. The pseudomyopia is the state of thickened lens as in refractive myopia before the instillation of a cycloplegia. After the instillation of a cycloplegia, however, the thickness of the lens becomes less and refractive error shifts toward the hyperopia side by 1 D (diopter) or more.
Clear distinction between these two kinds of myopia (axial myopia and refractive myopia), however, is very difficult to make and the coexistence of the two is said to be frequently seen. At present, it is commonly understood that the close work for the eye is one of the causes of myopia, though the cause of myopia still remains to be clarified. More specifically, when close work for the eye is done, contraction of ciliary smooth muscle makes the lens thicken. Continuation of this state for an extended period of time results in failure to restore to its original state, which in turn causes substrate change of ciliary smooth muscle to result in refractive myopia, which further causes fragile posterior membrane of the eye to permit growing axis of the eye, thus causing axial myopia.
Alternatively, alleviation of the contractile state of ciliary smooth muscle is said to cure myopia. For this end, local administration (instillation) of a drug such as tropicamide is done for alleviating the tension of ciliary smooth muscle. Yet, the efficacy of the drug differs among patients and is not entirely satisfactory. Besides the administration, physical therapy such as low frequency therapy, ultrasonic therapy and training of looking far-off are tried, none of which, however, has achieved satisfactory results.
As described in the above, there is no satisfactory method or composition for the prophylaxis and treatment of myopia. Thus, the strong need remains for the development of a superior composition and a method for the prophylaxis and treatment of myopia in both patients and doctors.